Technical Field
The present disclosure relates to a station side terminal device, a subscriber side terminal device, an optical communication system, a route switching method, a non-transitory computer readable medium storing a route switching program, and a wavelength switching method.
Related Art
Recently, a service known as fiber to the home (FTTH), which utilizes optical fiber as a transmission path, has come to be widely used with the objective of providing high speed, high bandwidth broadband services to general individual residences. Optical access networks known as passive optical networks (PON) are widely employed when providing broadband services by FTTH.
PON is configured by making a one-to-many connection between a single station side terminal device (an optical line terminal (OLT)) and plural subscriber side terminal devices (optical network units (ONU)) by splitting a single optical cable using passive optical elements known as optical splitters (optical couplers). PONs enable FTTH services to be provided economically by sharing an optical fiber, an OLT, and the like between plural subscribers.
Examples of PONs include what is known as a 10 Gigabit Ethernet (registered trademark) PON (10G-EPON) (for example, see IEEE (Institute of Electrical and Electronics Engineers) Std 802.3av-2009 (referred to as Document 1 hereafter)). In the PON described in Document 1, time division multiple access (TDMA) technology is employed in communication from ONUs to the OLT (upstream communication), and conflicts between signals from respective ONUs are avoided. PONs that employ such TDMA technology are known as TDM-PONs.
Moreover, to respond to increasing demand for communication over optical access networks, as next generation PONs having transmission rates exceeding 10 Gbps, research and development related to WDM/TDM-PONs (TWDM-PONs) is advancing in which plural TDM-PONs are configured on a single PON infrastructure using wavelength division multiplexing (WDM) technology (for example, see Japanese Patent Application Laid-Open (JP-A) No. 2011-055407). The transmission capacity of PON infrastructure may be increased by employing a TWDM-PON.
FIG. 6 illustrates a schematic diagram of the TWDM-PON disclosed by JP-A No. 2011-055407. As illustrated in FIG. 6, the TWDM-PON 1 is configured including an OLT 2 and plural ONUs: ONU 6-1 to ONU 6-m (where m is an integer of 2 or more). The OLT 2 is configured including a control device 3 that controls the TWDM-PON 1, plural optical subscriber units (OSUs) 4 (4-1 to 4-n, where n is an integer of 2 or more), and plural optical transceivers 5 (5-1 to 5-n) that are each connected to the respective OSUs 4. The optical transceivers 5 are each connected to the plural ONUs 6 through an optical splitter 7.
The optical transceivers 5 of the OLT 2 are each allocated a respective fixed reception wavelength for upstream communication so that reception wavelengths of the respective optical transceivers 5 of the OLT 2 do not overlap with one another. In such cases, respective connections between the optical transceivers 5 of the OLT 2 and the ONUs 6 can be dynamically switched by changing the transmission wavelengths of optical transceivers (not illustrated in the drawings) of the ONUs 6. The optical transceivers 5 of the OLT 2 are also each allocated with a respective fixed transmission wavelength for communication from the OLT 2 to the ONUs 6 (downstream communication). Similarly to upstream communication, and respective connections between the optical transceivers 5 of the OLT 2 and the ONUs 6 can be dynamically switched by changing the reception wavelength of optical transceivers of the ONUs 6. Accordingly, not only is bandwidth increased in the TWDM-PON 1, but merits thereof also include distribution of load according to variation in traffic, increased reliability due to route switching in the event of a failure, and reduced power consumption due to optical transceivers and device circuits sleeping in the event of low loads.
When dynamically switching connections between the OLT 2 and the ONUs 6 for downstream communication in the TWDM-PON 1, for example, switching of the optical transceivers 5 of the OLT 2 and switching of the reception wavelengths of the ONUs 6 are performed. Downstream communication packets (also simply referred to as “downstream packets” hereafter) cannot be received by an ONU 6 during a switching time over which the reception wavelength of the ONU 6 is being switched from a pre-switch wavelength to a post-switch wavelength. However, in multimedia applications and the like, in terms of service quality, it is preferable for no packet loss to arise during the switching time, there is accordingly demand for a switching process without any disconnection.
Accordingly, packets addressed to an ONU 6 targeted for switching need to be buffered in the OLT 2 during the switching time to avoid downstream communication packet loss during the switching time. For switching communication routes while buffering inputted packets, technology has been proposed in which a buffer is provided at an earlier stage than the switch that switches the route, and the route is switched by the switch according to the address of the inputted packets (for example, see JP-A No. H10-229404).
On the other hand, “Draft new Recommendation ITU-T (International Telecommunication Union Telecommunication Standardization Sector) G989.3 (for Consent, 4 Apr. 2014)” (referred to as Document 2 hereafter) discloses a wavelength switching sequence that is a procedure for dynamically changing transmission and reception wavelengths of the ONUs 6 in a TWDM-PON system like that described above.
More specifically, a procedure such as the following is executed. Namely, hereinbelow, in FIG. 6, a case in which the wavelength of the optical transceiver 5-1 connected to the OSU 4-1 is λ1, the wavelength of the optical transceiver 5-2 connected to the OSU 4-2 is λ2, the wavelength of the optical transceiver of the ONU 6-1 is designated as λ1, and the ONU 6-1 is changed from a state of being connected to the OSU 4-1 to being connected to the OSU 4-2, will be described.
When the above case is performed, the OLT 2 transmits a wavelength switching control message to the ONU 6-1 via the OSU 4-1, containing an instruction to switch wavelength to λ2. Having received the wavelength switching control message, the ONU 6-1 responds to the OSU 4-1 with a wavelength switching response message, and changes the wavelength of its own optical transceiver to λ2. After the wavelength change has completed, the ONU 6-1 transmits a wavelength switching completion message to the OSU 4-2, and having received the wavelength switching completion message, the OLT 2 recognizes that the wavelength switching of the ONU 6-1 has completed. Then, the OLT 2 switches the output route for downstream traffic addressed to the ONU 6-1 from the OSU 4-1 to the OSU 4-2 in the control device 3, and re-starts transmission of signals for the ONU 6-1.
In this TWDM-PON, the OLT identifies the downstream packets by ONU unit, and assigns the packets to the optical transceiver allocated with the transmission wavelength corresponding to the reception wavelength of the ONU at that point in time.
When the configuration described in JP-A No. H10-229404, mentioned above, is applied in order to perform route switching without disconnection in the TWDM-PON, there is a need to install, at an earlier stage than the switch, as many buffers as there are ONUs accommodated by the TWDM-PON. The circuit scale accordingly increases when there are many ONUs accommodated therein. Moreover, since capacity sufficient to hold the packets during the switching time is needed in each of the buffers, a large buffer size is needed when the switching takes a long time. Increases in the circuit scale and buffer size may lead to issues in device implementation.
Moreover, when a shared buffer system in which a buffer is shared is applied, address management information increases as the number of accommodated ONUs increases. A large amount of memory is therefore needed to address management when there are many accommodated ONUs, which may lead to an issue in device implementation.
Broadband services provided to general individual residences also include distributable broadcast services, and there is a need to be able to implement wavelength switching operations without causing service quality degradation, such as packet loss, during provision of broadcast services.
Ultra-high-definition video on demand (VOD) 4K video delivery services, which have started operation in recent years, are examples of such broadcast services, and hereafter the provision of, for example, 4K video broadcast delivery services that utilize the high bandwidth capabilities of TWDM-PON systems is expected. For example, a video delivery service is conceivable in which, when broadcasting multichannel 4K video, ch1 to ch100 are distributed using wavelength λ1, ch101 to ch200 are distributed using wavelength λ2, and video is received by selecting which wavelength and channel to receive at the ONU side.
However, in a TWDM-PON system that provides such a 4K video delivery service, when, for example, there has been a request to view ch101 from a subscriber under an ONU at wavelength λ1, in the wavelength switching sequence described in Document 2, wavelength switching control is made by control from the OLT at the discretion of the OLT. Thus wavelength switching cannot be implemented by the ONU, and the desired channel cannot be viewed. Moreover, when wavelength switching is freely implemented by the ONU, services other than the video delivery service are cut off, such as internet data communication received up to that point.